Integrated circuits are typically fabricated in monocrystalline silicon substrate. This substrate is expensive, leading to continual efforts to fabricate more circuitry within a given area of substrate, increasing density.
For nonvolatile memory, a highly effective approach to increase density is to build monolithic three dimensional memories above the substrate, like those disclosed in Johnson et al. U.S. Pat. No. 6,034,882; Johnson et al. U.S. patent application Ser. No. 09/928,536, filed Aug. 13, 2001, now U.S. Pat. No. 6,525,953; Knall et al. U.S. Pat. No. 6,420,215; and Vyvoda et al. U.S. patent application Ser. No. 10/185,507, filed Jun. 27, 2002, now U.S. Pat. No. 6,952,043, all hereby incorporated by reference in their entirety.
Such memories can be improved to achieve higher densities.